large crank length is bad for the knees, true/false?

[montage]

Anybody read or got some solid arguments for this debate?

 

[GrasB]

if the length is to long it'll be bad for your knee as you'll have to over extend it during one part of the stroke. When this happens or not is another matter, it may need a stupidly oversized crank.

 

[e-rider]

...only if you have short legs!!! It's all relative.

I would say short cranks are bad for the knees if you have long legs.

 

[accountantpete]

Well it flexes the knee joint a bit more but the increased leverage will put less pressure on the knee - swings and roundabouts then.

 

[MacB]

I've seen recommendations around shorter cranks being better for a spinnier cadence. Maybe it means longer cranks promote more of a grinding style? I'd imagine the cranks would have to be seriously mismatched to you to cause knee problems on their own.

 

[RecordAceFromNew]

James Martin, a professor at the University of Utah did a lot of research on this subject, see this e.g. The conclusion is that it doesn't make much difference... which is not dissimilar to Sheldon's observation, who I believe said that we all climb stairs of different pitch, ride using different gearing with hardly a second thought.

 

[Steve Austin]

I've used long cranks of 180 for a while and it just meant i found it harder to spin freely.
I think the theory goes that if you are riding with long cranks its harder to spin so riders tend to mash the gears which is rumoured to be bad for the knees.

I don't think there is any science either way, just a lot of speculation.

 

[tyred]

Old roadsters like my Rudge had longer than usual cranks and high gearing but the people that rode them back in the day never came to any harm. I find mine favours a slower pedaling style than what you would use on a road bike but I never feel I'm stressing my legs in any way. The extra leverage is useful on a heavy bike.

 

[Fiona N]

I used shorter cranks on the Speedmachine to help with spinning and getting the pedals round on steep hills in low gear when the cadence dropped. But I've never seen the point on normal uprights - it makes the apparent gear higher, for a start.

The only place I've come across longer than usual cranks was, bizarrely, on a gym bike in Tokyo - given that most of the users are short older men (< 5'6") with shorter legs, this seemed like asking for trouble. The cranks were so much longer than normal that I noticed the difference and got off the bike to check - they must have been at least 200mm

 

[jimboalee]

I used shorter cranks on the Speedmachine to help with spinning and getting the pedals round on steep hills in low gear when the cadence dropped. But I've never seen the point on normal uprights - it makes the apparent gear higher, for a start.

The only place I've come across longer than usual cranks was, bizarrely, on a gym bike in Tokyo - given that most of the users are short older men (< 5'6") with shorter legs, this seemed like asking for trouble. The cranks were so much longer than normal that I noticed the difference and got off the bike to check - they must have been at least 200mm

That's so they are exercising over a larger range than normal.

 

[jimboalee]

I've used long cranks of 180 for a while and it just meant i found it harder to spin freely.
I think the theory goes that if you are riding with long cranks its harder to spin so riders tend to mash the gears which is rumoured to be bad for the knees.

I don't think there is any science either way, just a lot of speculation.

I think it is, as you say, merely speculation.

What is 'bad' for any part of the body is exerting it at an intensity far above what is normal without progressing to that level gradually at the rate of physiological adaptation.

Cyclists who normally spin a short crank might get on a long crank bike and grind it up a hill. They are not trained for this sudden change in intensity, so they suffer.

I'm the opposite, spinning at 100+ rpm gives my knees the shakes because I'm trained for 70 - 80 rpm.

It is beneficial for all cyclists to train for all circumstances, ie, if you never grind, find a shallow hill and do it. Recover and then do it again harder. Recover and repeat, getting harder each session.
Then when you get to a hill with your group of friends, will they be surprised?

 

[ASC1951]

What is 'bad' for any part of the body is exerting it at an intensity far above what is normal without progressing to that level gradually at the rate of physiological adaptation.

Cyclists who normally spin a short crank might get on a long crank bike and grind it up a hill. They are not trained for this sudden change in intensity, so they suffer.

That's certainly true for muscles but not for other components such as joints and ligaments. Repeated stress beyond their design limits does cause damage, irrespective of the amount of training - look no further than the knees of professional footballers, the ankles of professional dancers or the kidneys of professional cyclists.

I've used cranks from 165s to 175s and haven't found any huge difference except when riding fixed on the road; then you need the extra leverage because you can't compensate by gearing down for hills.

 

[Mycroft]

or the kidneys of professional cyclists.

please explain?

 

[jimboalee]

That's certainly true for muscles but not for other components such as joints and ligaments. Repeated stress beyond their design limits does cause damage, irrespective of the amount of training - look no further than the knees of professional footballers, the ankles of professional dancers or the kidneys of professional cyclists.

I've used cranks from 165s to 175s and haven't found any huge difference except when riding fixed on the road; then you need the extra leverage because you can't compensate by gearing down for hills.

Agreed, but a bike only weighs 25lb. If I put a washing machine on your back and asked you to carry it up a dozen flights of stairs, I would expect you to suffer ligament damage.

Ride a bicycle up a 25% incline and your muscles will scream well before tendons, ligaments or Synovial joints.

 

[buddha]

I've only ever used 170mm cranks. But if CRC deliver tomorrow one bike will have 152mm cranks (due to pedal strike issues after changing to narrow slicks on an mtb).

I suppose it will feel strange at first. But, after reading this, may improve my spinning?